The present invention relates to a cathode for a fused salt reduction cell used in the production of aluminum wherein the cathode is chracterized by wettable work face surfaces which are resistant to the electrolyte and are electrically conductive.
In the production of aluminum by fused salt electrolytic reduction of aluminun oxide the aluminum oxide is dissolved in a fluoride melt made up for the most part of cryolite. The cathodically precipitated aluminum collects on the carbon floor of the cell below the fluoride melt, the surface of the molten aluminum or a solid body which can be wetted by molten aluminum forming the cathode. Dipping into the electrolyte from above are anodes which are secured to an overhead anode beam. In conventional fused salt reduction processes the anodes are made up of amorphous carbon. Oxygen is produced at the anodes as a result of the decomposition of the aluminum oxide; this oxygen combines with the carbon in the anodes to form CO.sub.2 and CO.
The electrolytic process generally takes place at a temperature range of 940.degree.-970.degree. C. During the course of the process the electrolyte becomes deplete in aluminum oxide. At a concentration lower than approximately 1-2 wt.% aluminum oxide in the electrolyte an anode effect occurs where the voltage increases from for example 4-4.5 V to 30 V and higher.
It is known that a high current levels the combined effect of the vertical components of the magnetic field and the horizontal components of the current can lead to an undesired distortion on the surface of and some centimeters below the surface of the metal bath and to an undesired pronounced stirring of the metal bath. When the interpolar spacing is small, these distortions can be large enough to cause the aluminum to touch the anodes which results in short circuiting.
Furthermore, the stirring of the metal at the surface thereof can lead to increased chemical dissolution or fine dispersion of aluminum in the melt which, as is well known, results in a lower yield due to re-oxidation of the metal.
A lower current density is in theory advantageous, however, in reality would incur unacceptably high increases in the capital costs for the reduction pots and for the pot room.
Cathodes which can be wet by aluminum have been known for some time now. A characteristic of these cathodes is a thin layer of aluminum which can move only a little in the direction vertical to the work face. As a result, the classical surface distortion effects, that is, both stationary doming and moving waves, can to a large extent be eliminated. Apart from the very high material costs, however, the foregoing arrangement, with reduced interpolar spacing, suffers from the disadvantage that the circulation of electrolyte between anode and cathode is made more difficult as a result of which the cryolite melt becomes deplete in alumina due to the precipitation of aluminum and, therefore, the cell is prone to exhibit the above-noted anode effect. Two ways of improving the circulation of electrolyte were attempted in the prior art. In the first case the solid cathodes do not extend their whole surfaces, at a uniform spacing from the anodes over the whole work area thereof. Rather, individual parts of the cathode project upwards towards the anodes and/or slits are provided in the surface of the cathode. In the second case a pourable, particulate bulk material is introduced into the cell such that the bulk material is completely covered by the molten metal. In addition to the foregoing, U.S. Pat. Nos. 3,661,736 and 4,308,114 disclose the use of a solid composite cathode for an aluminum fused salt reduction pot. In these cases, refractory particles of a material which is wet by aluminum are embedded in a carbon matrix.
The first of the above mentioned patents proposes manufacturing the composite by mixing a fine carbon powder with granular titanium diboride and therefore treating the mix in a suitable thermal process. In the second patent, granular titanium diboride is mixed into tar or pitch. Such composite cathodes are only capable of being slightly wet by aluminum; the carbon matrix comes into contact with the electrolyte. The interpolar gap can be reduced at most to about 4 cm.
In light of the foregoing it is the principal to develop cathode for an aluminum molten salt reduction cell which is completely wet by aluminum, is not attacked by the electrolyte, can be manufactured economically and is easy to replace.